Method and device for sensing a clearance

ABSTRACT

A method and an arrangement for sensing the clearance between an object and an object-adjacent surface, which is particularly for use in difficult ambient conditions, such as in dirty, corrosive environments and under varying temperature and pressure conditions, e.g. to measure and hold constant the clearance between the sector plates ( 3, 4 ) of a regenerative rotary air preheater and the end surfaces of the rotor ( 2, 8 ). The sensing device ( 7 ) includes a compressed-air operated pipe ( 9 ) which is mounted on the object ( 3, 4 ) and which includes a sound-emitting opening ( 14 ) located adjacent the surface ( 8 ), wherein changes in the distance (S) between the object ( 3, 4 ) and the surface ( 8 ) are represented by changes that occur in the resonant frequency of the pipe ( 9 ).

BACKGROUND OF THE INVENTION

The present invention relates to a method of sensing the clearancebetween an object and a surface that lies adjacent thereto, and ofgenerating a signal representative of the magnitude of said clearancefor adjusting the clearance to a desired magnitude with the aid of anadjusting device controlled in dependence on said signal.

The invention also relates to an arrangement by means of which themethod can be carried out.

Clearances ranging from some tenths of a millimetre up to severalmillimetres can be measured in many ways. The measuring process applied,however immediately becomes more complicated when it is effected inorder to generate a signal that controls a setting device for adjustingthe clearance. This is particularly true of measuring processes that areeffected in a troublesome atmosphere, such as in the case ofregenerative rotary heat-exchangers that operate in a corrosiveatmosphere contaminated with soot for instance, and under high, varyingpressure and temperature conditions.

In an application of this nature, the clearance between moving sectorplates or seals and a rotor surface must be held constant, said rotorbeing through-passed axially by air and combustion gases for instance.

As will be evident from U.S. Pat. No. 3,246,686. Mobile mechanicalsensors have been tested, although ambient conditions have been foundtoo troublesome with respect to bearings and journals among otherthings.

The same applies to the devices disclosed in U.S. Pat. No. 3,232,335.These devices include on the one hand a sliding shoe that slides againstthe rotor and on the other hand an inductive component and acompressed-air jet for contactless sensing of a clearance. Dirtying ofthe slide shoe and its bearings makes the use of this solutionimpossible in practice. A corrosive atmosphere and high temperaturesmake the use of an inductive device impossible in practice and themarkedly varying pressure conditions in particular have a pronouncedeffect on measuring processes that use a jet of compressed air.

Sliding shoes made of ceramics and carbon or graphite have been proposedin recent years. However, these shoes result in higher friction and inproblems relating to lubrication of contacting surfaces, subsequentadjustment of clearances, among other things. The use of air cushionshas been proposed as an alternative to sliding shoes. The use of aircushions, however, introduces complications, partly as a result of thegreatly varying pressure conditions. A sensing device that includesfibre optics for infrared light beams has also been proposed (U.S. Pat.No. 4,306,612). Such sensing devices are troubled by corrosion, dirtcontamination, etc., and do not therefore provide a satisfactorysolution.

The object of the invention is to provide a method and an arrangement ofthe kind defined in the introduction that will enable clearance rangingfrom some tenths of a millimetre to several miliimetres to be measuredsimply and positively in all relevant ambient conditions.

This object is achieved with the method comprising the method steps setforth in claim 1 and with the arrangement that has the characteristicfeatures set forth in claim 2

SUMMARY OF THE INVENTION

The invention is based on the understanding that a whistle pipe, organpipe or flute can be tuned to an exact resonance frequency and that theso-called end correction and the resonance frequency will changemarkedly when an object is brought into the close vicinity of asound-emitting opening of the pipe or flute, while a change in thelength of the pipe or flute will cause the resonance frequency to changein accordance with the change in length. Hence, by identifying andutilizing said frequency, it is possible to control a setting device foradjusting a clearance. Trials have shown that a frequency change of 50Hz can correspond to a change in clearance of from 2 to 3 mm. Sincefrequencies of some few Hz can be measured readily, it will beunderstood that the invention affords an accuracy that has not earlierbeen achieved. The only driving force required to operate the pipe is aweak compress-air flow. One particular advantage afforded by theinvention is that the frequency is highly insensitive to external orambient conditions, which although affecting the strength of the soundthat is generated have no affect on the resonance frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference toexemplifying embodiments of the invention and with reference to theaccompanying schematic drawings, of which

FIG. 1 shows part of an air-preheater from above;

FIG. 2 is a partly sectioned side view of a sector-plate mountedarrangement for contactless measuring of the clearance to a rotor;

FIG. 3 is a longitudinal section view of an open pipe included in thearrangement shown in FIG. 2;

FIG. 4 is a graph that shows measured frequency as a function ofclearance; and

FIG. 5 is a partly sectioned side view of another embodiment of theinvention.

DETAILED DESCRIPTION

FIG. 1 shows a typical rotary regenerative heat exchanger which isintended for preheating combustion air with the aid of exhaust gases andwhich includes a stationary housing 1 and a rotor 2 that has aregenerator mass and is mounted for rotation in the housing 1. Twoobjects comprising axially movable sector plates 3, 4 are mounted onbearings above and beneath the rotor and close to its end surface. Thesector plates 3, 4 separate a gas side 5 from an air side 6, wherewithalthough gas and air is able to enter the clearance beneath respectivesector plates 3, 4 due to rotation of the rotor, no direct leakage needoccur from one side to the other provided that the clearance between thesector plates 3, 4 and the ends of rotor can be kept small in spite ofthe fact that the ends of the rotor will never be completely flat.

The outer ends of the sector plates 3. 4 are guided by sensing devices 7that coact with an object-adjacent surface comprising a circular flange8 extending around the rotor circumference at the top and at the bottomof the rotor 2, as evident from FIG. 2.

The sensing devices 7 comprise of a compressed-air operated pipe 9, ofthe whistle pipe, flute or organ pipe kind, that has a sound-emittingend opening 14 and that has been tuned to a specific frequency. Eachsensing device 7 is attached to a lug 10 that projects out from eachside of a respective sector plate 3, 4 immediately above the upper rotorflange 8 and immediately beneath the lower rotor flange (not shown). Thebottom part of the pipe 9 is threaded and screwed into a hole providedin the lug 10, and locked with a lock nut 11.

The sector plates 3, 4 are set by means of a setting device 12 which isfixedly attached to the housing and has an adjuster rod 13 connected toa respective sector plate, said sector plates being set with theunderside about 2 mm from the rotor end and the flange 8.

The pipe 9 has the well-known form shown in FIG. 3, including a straighttube in which a so-called mouth opening 15 has been made. A block 16that includes an air passageway 17 is inserted edge to edge with theupper edge of the opening 15. Provided at the upper end of the pipe 9 isa nozzle 18 that includes a compressed-air passageway 19 that opens intoa nipple 20 which is connected to a hose 21 that leads to a source 21 aof compressed-air.

A sleeve 22 is mounted around part of the pipe 9 and the nozzle 18 andfunctions as an attachment means for a hose lead 23 that communicateswith the sleeve interior, where the mouth opening 15 is located, bymeans of passageways not shown. The hose lead 23 has a transit orlead-through 24 for the hose 21 and is connected to a resonancefrequency indicating device comprising a microphone 25 which, in turn,is connected to a frequency measuring device 26 that includes anecessary filter and amplifier. The frequency measuring device isdesigned to control the setting device 12 so that if the clearance Sbetween the underside of respective sector plates 3, 4 and the rotorflange 8 should decrease, the frequency measuring device 26 will delivera first signal that causes the setting device 12 to lift the sectorplate 3, 4 slightly. If the clearance S should increase, the frequencymeasuring device will deliver a second signal that causes the settingdevice 12 to lower the sector plate 3, 4 slightly.

This function of the sensing device 7 is based on the fact that when asurface approaches the end opening 14 of the pipe 9, the end correctionof the resonance tube will be affected within a distance equal to 0.6times the end diameter of said tube, therewith resulting in an increasein the resonance frequency or a decrease in the resonance frequency ifthe distance should decrease.

FIG. 4 shows how the frequency is changed when the gap A between the endsurface 14 of the pipe 9 and the rotor flange 8 is changed between 1 mmand 6 mm in a certain case.

FIG. 5 shows a modified version of the sensing device. In this case, thebottom part of the pipe 9 has the form of a bore 30 drilled in a carbonor graphite block forming an end sleeve 31 and thus forming acontinuation of the bore in the pipe 9. The block has an open end that31 is arranged to slide on the flange 8 and the pipe 9 therefore forms aclosed pipe with the singularity of initially being tuned to a resonancefrequency that is dependent on the length of the pipe 9. When wearoccurs on the end sleeve 31, the pipe 9 will have a shorter length andthe frequency therefore increase. The frequency is measured in the sameway as that described with reference to FIG. 2 and when the frequencyhas increased by a value corresponding to 2 mm for instance, the sensingdevice causes a block-actuating setting device 32 attached to the sectorplate 3, 4 to move the end sleeve 31 down through 2 mm for instance.This procedure is repeated until the end sleeve 31 has been almostcompletely consumed and must be replaced.

In this embodiment it is possible to utilize the fact that if for somereason the block 31 should become distanced from the flange 8, thefrequency will be increased by a full octave so as to produce apronounced signal.

It will be understood that the invention is not restricted to theillustrated and described embodiments thereof and that modifications canbe made within the scope of the inventive concept as defined in thefollowing claims. This applies particularly to the measuring equipment.The mouth opening 15 of the pipe 9 may be used to measure the clearance,instead of the end openinigs 14 of the pipe 9. When required, pressureand temperature variations can be compensated for in a known manner. Byplacing the sensing devices 7 on outwardly projecting peripheralfastener lugs 10, essentially constant pressure and temperatureconditions can be achieved in normal operation of a regenerative rotaryheat exchanger, by virtue of the sensing devices 7 therewith beinglocated on the air side 6 and the gas side 5 respectively.

What is claimed is:
 1. A method of sensing a clearance between an objectand an object-adjacent surface, and of generating a signal which isrepresentative of a magnitude of said clearance and which is used tocontrol a setting device that is operated to adjust the magnitude ofsaid clearance to a desired magnitude, the method comprising: providingthe object with at least one compressed-air operated pipe that includesa resonance tube that is tuned to a specific frequency and that includesan opening which is adapted to be placed adjacent to saidobject-adjacent surface such that a resonance frequency of the at leastone pipe changes in accordance with the magnitude of the clearance; andidentifying and utilizing said resonance frequency to control thesetting device to adjust the magnitude of said clearance to the desiredmagnitude.
 2. An arrangement for carrying out the method of sensing aclearance between an object and an object-adjacent surface in accordancewith claim 1, wherein: the object is provided with a sensing device thatcomprises the at least one pipe and that is operated usingcompressed-air taken from a compressed-air sources; the at least onepipe is tuned to a resonance frequency and has at least onesound-emitting end opening; the at least one pipe is positioned with theat least one sound-emitting end opening located adjacent to saidobject-adjacent surface so that the resonance frequency of the at leastone pipe is dependent on the magnitude of the clearance between theobject and the object-adjacent surface; and the setting device isadapted to be controlled in accordance with an output from a resonancefrequency measuring device that identifies the resonance frequency ofthe at least one pipe.
 3. An arrangement according to claim 2, whereinthe at least one pipe comprises a straight open pipe which is mounted onthe object at right angles to said object-adjacent surface with a gapbetween the object-adjacent surface and the at least one sound-emittingend opening of the at least one pipe.
 4. An arrangement according toclaim 2, wherein the at least one pipe comprises a straight open pipewhich is mounted on the object and which has an open end which isextended by an end sleeve made of at least one of carbon and graphite,and wherein said end sleeve forms a continuation of the at least onepipe in a manner such that an open end of the end sleeve is arranged toslide on the object-adjacent surface and the object-adjacent surfacecloses said open end of the end sleeve.
 5. An arrangement according toclaim 2, wherein: the object comprises a sector plate on a regenerativerotary heat exchanger; the object-adjacent surface comprises aperipheral flange on a rotor of said heat exchanger; and the sectorplate is provided on at least one side thereof with a part whichprojects out peripherally opposite the peripheral flange of the rotorand which carries the sensing device.
 6. An arrangement according toclaim 3, wherein: the object comprises a sector plate on a regenerativerotary heat exchanger; the object-adjacent surface comprises aperipheral flange on a rotor of said heat exchanger; and the sectorplate is provided on at least one side thereof with a part whichprojects out peripherally opposite the peripheral flange of the rotorand which carries the sensing device.
 7. An arrangement according toclaim 4, wherein: the object comprises a sector plate on a regenerativerotary heat exchanger; the object-adjacent surface comprises aperipheral flange on a rotor of said heat exchanger; and the sectorplate is provided on at least one side thereof with a part whichprojects out peripherally opposite the peripheral flange of the rotorand which carries the sensing device.